Advice on dildo size - present for girlfriend

As part of my fantasies about watching my girlfriend with a bigger man inside her, we have been looking into trying a dildo (she has never used one before).

We did order the Doc Johnson Bam Black Realistic Cock 10.5 Inch (http://www.lovehoney.co.uk/product.cfm?p=12107), but poorly estimated the size - all she did was laugh, and we sent it back without trying. That was seriously too big - she is quite a petite girl, poor thing.

I really want to try something bigger than me, but not too much! I did an honest measurement of myself which comes up as 7" length and 6" circumference. One problem I find is that anything over 6" circumference seems to have a huge 10 - 12 inch length. I think she would enjoy the extra girth (though not the Bam Realistic girth of 8.5!) but more than 8 or 9 inches would probably put her off.

Any recommendations? Any tips on whether this is a good idea to get my girlfriend into big boys?

hubby bought unfortunaty from shop as he was working away so we wanted it quick ! so the one he got was called michelles mammouth not sure which shop this is 8 inch circumferance hope this helps if you can ask on phone for one this thick not painful and doable